1. Field of the Invention
The present application is directed to plasma torches and, more particularly to a plasma torch having interchangeable electrode systems such that the same plasma torch is capable of efficiently cutting both thinner and thicker workpieces.
2. Description of Related Art
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treating, melting, and annealing. Such torches include an electrode which supports an electric arc that extends from the electrode to a workpiece. A plasma gas is typically directed to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. In some torches, a second or shielding gas, or a swirling jet of water, is used to surround the jet of plasma gas and the arc for controlling the work operation. One characteristic of existing plasma arc torches is that there is little or no efficient commonality between torches or torch configurations used to cut relatively thinner workpieces and torches or torch configurations used to cut relatively thicker workpieces. Thus, a user who desires to cut both thinner and thicker workpieces must often purchase two complete and different torch assemblies. Furthermore, a plasma arc torch manufacturer who desires to make both types of torches must manufacture and maintain inventories of two complete sets of different components, and therefore the cost complexity of the manufacturing operation are increased when both types of torches are involved. If a torch is capable of cutting both thinner and thicker workpieces, the operating conditions of such a torch for cutting a thicker workpiece may not be desirable in terms of, for example, efficiency. For instance, a Model PT-15 torch manufactured by The ESAB Group, Inc. is one example of a torch capable of cutting both thin and thick plate materials. However, cutting plates as thick as, for example, 6 inches, requires such a torch to operate at a current level of 1000 amperes, a gas flow of 400 scfh, and a voltage of up to 250 volts. Accordingly, such operational parameters make a thick plate cutting operation a relatively cost-intensive undertaking.
In a typical plasma arc torch, the plasma gas and a shielding gas or water are directed by a nozzle assembly having a plasma gas nozzle and the shielding gas or water injection nozzle coaxially arranged concentrically or in series. The nozzle assembly is electrically conductive and is insulated from the electrode so that an electrical potential difference can be established between the electrode and the nozzle assembly for starting the torch. To start the torch, one side of an electrical potential source, typically the cathode side, is connected to the electrode and the other side, typically the anode side, is connected to the nozzle assembly through a switch and a resistor. The anode side is also connected in parallel to the workpiece with no resistor interposed therebetween. A high voltage and high frequency are imposed across the electrode and nozzle assembly, causing an electric arc to be established across a gap therebetween adjacent the plasma gas nozzle discharge. This arc, commonly referred to as a pilot or starting arc, is at a high frequency and high voltage but a relatively low current to avoid damaging the torch. Plasma gas is caused to flow through the plasma gas nozzle to blow the pilot arc outward through the nozzle discharge until the arc attaches to the workpiece. The switch connecting the potential source to the nozzle assembly is then opened, and the torch is in the transferred arc mode for performing a work operation on the workpiece. The power supplied to the torch is increased in the transferred arc mode to create a cutting arc which is of a higher current than the pilot arc.
In some plasma arc torches, an emissive insert-type electrode is used for creating the arc from the electrode to a workpiece. Some such electrodes include, for example, a copper holder having a silver separator held in the copper holder. A hafnium emissive element or insert is held within the silver separator. Typically, the copper holder is held in the torch by way of external threads that mate with the internal threads of an electrode holder. Such a torch using an emissive insert-type element is generally known to be effective in cutting relatively thinner materials such as, for example, carbon steel plate up to about 1½ inches thick. In some instances, such as when cutting a thicker metal workpiece, a torch using a hafnium emissive element is usually not suitable since such a configuration is limited, for example, to a maximum current of about 400 amps. However, a torch using a tungsten insert in place of the hafnium insert in the holder can be used to cut thicker materials, though such a torch configuration using a tungsten insert electrode generally requires a minimum current of about 1000 amps in order to cut 6 inch thick material. Configuring such a torch to operate at such a high current level undesirably results in concerns regarding, for example, safety, operating efficiency, and cost of construction.
Other plasma arc torches, such as a torch using a tungsten pencil-type electrode, are generally known to be useful for cutting thick materials. Such tungsten pencil electrodes are formed of, for example, thoriated tungsten formed into a solid pencil-like shape that is held within the torch with a particular electrode holder arrangement. However, tungsten pencil-type electrodes cannot be used with air or oxygen (as the plasma gas) typically used with emissive insert-type electrodes. Instead, such tungsten pencil-type electrodes are commonly used with a mixture of 35% hydrogen and 65% argon, at up to about 600 amps for cutting thick plate materials, or with nitrogen and at currents below about 150 amps for cutting thinner plate materials. However, nitrogen and the mixture of 35% hydrogen and 65% argon are generally not the preferred gases for cutting steel less than about 1½ to 2 inches thick.
In summary, existing plasma arc torches are subject to several disadvantages such as, for example, lack of efficient commonality between torches or torch configurations used to cut relatively thinner workpieces and torches or torch configurations used to cut relatively thicker workpieces. Thus, there exists a need for a plasma torch capable of cutting both thinner and thicker plate materials in an efficient manner.